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II VXDBOOK OF PHYSIOLOGY 



M I R( il'in SI( II.OGV III 



1 Physiologic Hypothesis 



The phenomenal identity of apparent (optimal) 

 and real motion suggested to WVnheimer (5:56) and 

 to others after him, including Kohler (268) and Hel- 

 son & King (198), that whatever the stimulus for 

 any form of motion perception might be, it need not 

 be .1 continuous movement across the receptor surface. 

 Instead Wertheimer postulated a central correlate 

 — a continuous - phi'-process within, presumably, 

 the visual cortex; there, points a and b are repre- 

 sented in correspondingly separate loci, a' and b', 

 according to the principle of retinotopical projection. 

 At appropriate time intervals the excitation in a' 

 is "drawn over' to the excitation in //; corresponding 

 u> this 'physiological short circuit,' we perceive 

 movement from a to b. Analogous assumptions can 

 be made for tactile motion perception (utilizing the 

 principle of somatotopic projection) and for auditory 

 movement (by assuming that certain time differences 

 in the auditory system are translated into spatial 

 separation). The postulate of a psychoneural corre- 

 spondence in motion perception formed the core 

 of Kohlcr's isomorphism (267, 272) which assumes a 

 general correspondence in the spatial and temporal 

 older of perceptions and their underlying neural 

 (cerebral) events. As Bartley has discussed in Chapter 

 XXX of this Handbook, Kohler has attempted to 

 tesl this hypothesis by recording d.c. potentials from 

 the occipital scalp while the subject sees a pattern 

 move across his visual field (277-280). The theory of 

 isomorphism has no particular difficulty in accounting 

 for apparent (and real l movement across the normal 

 blind spot (44b). In this case, the retinal substrate 

 is discontinuous, but the cortical substrate is mil 

 (20). The theory, however, is embarrassed by the 

 In 1 that apparent motion can be obtained across 

 acquired scotomata, i.e. blind spots produced by 

 cerebral lesions (474). We shall return to these 

 observations later; the) bear particularly on the phe- 

 nomenon of "real' motion. 



PERCEP I 11 in hi RE \l Ml H H is 



li 11 is true that 'apparent' motion represents a 

 limiting ease ni 'inie' moiiini, ilien the distinction 

 between the iuu is unfortunate. The distinction has 

 led to a relative neglei 1 oi true motion, so that quan- 

 titative information on perception ol true motion is 

 sparse; where available, data are Limited b\ failure 

 to surve) a sufficient range of stimulus conditions 



Characteristics in Normal Subjects 



MINIMAL rates. A few experiments bear on the ques- 

 tion of thresholds for motion perception. There is a 

 minimal rate of displacement for a moving stimulus 

 below which observers may note a change, but not 

 real motion. Aubert (14, 15), under rather restricted 

 conditions, found a minimal rate of 1 to 2 min. of 

 arc per see.; later, Brown (66-68) obtained values 

 between 2 and 3 min. of arc per sec. under somewhat 

 less favorable conditions. Such thresholds obviously 

 vary with size and pattern of moving stimulus, with 

 pattern of surround, with illumination and with 

 numerous other factors. 



Surprisingly, there are not even adequate data for 

 minima] rates in peripheral as against central parts 

 of the retina, and this in spite of the general belief 

 that motion detection in peripheral regions of the 

 visual field is disproportionately better than other 

 forms of acuity. Likewise there is no reliable informa- 

 tion on thresholds for visually perceived acceleration. 

 If motion thresholds are defined as the minimal angu- 

 lar distance traversed (with rate held constant 1, 

 values can be cited that suggest a minimal detectable 

 extent of 20 sec. of arc (22). This is less than the 

 minimum angle for the resolution of two acuity 

 objects, a result anticipated by Exner in 1875 ( IJ 9)- 

 Others, however, such as Gordon (163), have re- 

 ported that values for displacement and for resolu- 

 tion thresholds are similar, at least under dim illumi- 

 nation. The issue remains unresolved, in the absence 

 of Studies for a wider range of rates, illuminations 

 and retinal positions. For the same reasons, we cannot 

 be certain whether lower vertebrates might have 

 better capacity for motion detection than man, as 

 Honigmann (223) has claimed for toads. 



DIFFERENTIAL THRESHOLDS, The thresholds for dis- 

 crimination of two different rates of motion had not 

 been determined directly until the recent studies ol 

 l.kman & Dahlback (111), although apprehension 

 of differential speeds are involved in so-called motion 

 parallax investigated by von flclmhollz (501, 502) 

 and others (56, 1(17. 512). Thus, von Tschermak- 

 Seysencgg (512), improving upon an earlier set-up 

 li\ Bourdon (56), determined the minimal detectable 



distance between two pins along the line of regard 

 when the observer looked al them with fixed ga/e 

 Or with moving head and eyes. Under the second 



condition, there is differentia] angular movement of 

 the pins (relative to the observer's eye) .is long as the 

 pins are ai different distances from the eve. I he aver- 



